Defrost cycle initiation system

ABSTRACT

A defrost cycle initiation system for refrigeration apparatus (e.g., a refrigerator, a refrigerator-freezer, or a room air conditioner) in which operation of the defrost system is initiated by frost build-up on the refrigeration apparatus. Generally, the refrigeration apparatus has cooling means for absorbing heat from a refrigerated zone, this cooling means being subject to the build-up of frost. The refrigeration apparatus has a main flow path for the intake of air from the refrigerated zone and for passage of the air over the cooling means to chill the air, and for discharge of the chilled air into the refrigerated zone, and a blower for forcing air through the main flow path. The defrost cycle initiation system of this invention includes an auxiliary flow path providing communication between the main flow path and the refrigerated zone, air normally flowing in the auxiliary flow path from the refrigerated zone to the main flow path when the refrigeration apparatus is in its normal mode of operation. The system further includes means for sensing the temperature of air flowing through the auxiliary flow path. The sensing means initiates a defrost cycle upon sensing a predetermined initiation temperature of air. Means are also provided for effecting reversal of flow of air in the auxiliary flow path in response to build-up of frost on the cooling means thereby to direct air having a temperature less than the abovementioned predetermined initiation temperature through the auxiliary flow path to lower the temperature of the air sensed by the sensing means below the predetermined temperature and for reestablishing the flow of air in the auxiliary flow path from the refrigerated zone to the main flow path upon the clearing of the cooling means of frost.

United States Patent [191 Shepherd 3,845,637 Nov. 5, 1974 DEFROST CYCLElNITIATlON SYSTEM Glen C. Shepherd, Garland, Tex.

Texas Instruments Incorporated, Dallas, Tex.

Filed: Sept. 6, 1973 Appl. No.: 394,947 I lnventor:

Assignee:

[52] US. Cl 62/155, 62/156, 62/180,

62/186, 62/419 Int. Cl. F25d 21/06 Field of Search 62/140, 156, 155,180,

References Cited UNITED STATES PATENTS 12/1966 Kelly..... 62/180 3/1967Jacobus 2/1972 Harrison. 1/1974 Schrader 62/262 Primary ExaminerMeyerPerlin Attorney, Agent, or Firm.lohn A. Haug; James P. McAndrews; EdwardJ. Connors, Jr.

[5 7 ABSTRACT eration apparatus. Generally, the refrigeration apparatushas cooling means for absorbing heat from a refrigerated zone, thiscooling means being subject to the build-up of frost. The refrigerationapparatus has a main flow path for the intake of air from therefrigerated zone and for passage of the air over the cooling means tochill the air, and for discharge of the chilled air into therefrigerated zone, and a blower for forcing air through the main flowpath. The defrost cycle initiation system of this invention includes anauxiliary flow path providing communication between the main flow pathand the refrigerated zone, air normally flowing in the auxiliary flowpath from the refrigerated zone to the main flow path when therefrigeration apparatus is in its normal mode of operation. The systemfurther includes means for sensing the temperature of air flowingthrough the auxiliary flow path. The sensing means initiates a defrostcycle upon sensing a predetermined initiation temperature of air. Meansare also provided for effecting reversal of flow of air in the auxiliaryflow path in response to build-up of frost on the cooling means therebyto direct air having a temperature less than the above-mentionedpredetermined initiation temperature through the auxiliary flow path tolower the temperature of the air sensed by the sensing means below thepredetermined temperature and for reestablishing the flow of air in theauxiliary flow path from the refrigerated zone to the main flow pathupon the clearing of the cooling means of frost.

13 Claims, 15 Drawing Figures PATENIEMV 51924 $845637 I SEE! k (If 6FHQIO FIG.7

1 DEFROST CYCLE INITIATION SYSTEM BACKGROUND OF THE INVENTION Thisinvention relates to a defrost cycle initiation system for various typesof refrigeration apparatus, and more particularly to such systems whichare operable in response to the actual build-up of frost on the coolingunit (e.g., the evaporator) thereof.

The accumulation or build-up of frost on the evaporator of refrigerationapparatus and its efficient re moval have long presented seriousproblems. Various automatic defrost cycle initiation systems have beenused and are well known in the art but have not satisfactorily resolvedthese problems. For example, in room air conditioners the rate of frostbuild-up on the evaporator depends on a number of factors, including theroom ambient temperature and humidity conditions. Under severe frostconditions, air flow through the evaporator of an air conditioner maybecome blocked (or partially blocked) thus choking the flow of chilledair into the room, reducing the heat transfer efficiency of theevaporator to the point where the room cannot be adequately cooled, andreducing the operating efficiency of the unit such that excessive powermay be consumed. In some room air conditioners, however, no automaticdefrost system as such is provided, but rather the normal on-off cyclingof the unit under the control of the room temperature cold controlallows the evaporator to clear itself of frost when the compressor isnot operating. These units may work satisfactorily under certainconditions, but if the evaporator cannot clear itself of frost betweencompressor run cycles, frost may continue to build-up on the evaporatorand may eventually block the flow of air through the evaporator. In theevent this happens, the compressor will continue to run because the roomcan no longer be adequately cooled and the only recourse is to manuallyshut off the air conditioner until the frost melts from the evaporator.

On many room air conditioners now in use. various thermal sensingsystems are used to detect the presence of frost on the evaporator andto initiate a defrost cycle by deenergizing the compressor whilecontinuing to draw room air into the unit for passage over theevaporator and discharge back into the room. Such room air conditionersutilize in their defrost initiation systems a temperature-responsive,fluid-expansion bulb (e.g., a capillary tube) spaced in front of theevaporator a predetermined distance by frost clips or by other mountingmeans so as to sense the temperature of the room air as it is drawn intothe unit and to sense the build-up of frost on the evaporator. Upon thecapillary tube being cooled to a predetermined temperature (either byroom air impinging on the capillary tube or by increased heat transferfrom the capillary tube to the evaporator via the frost clips asoccasioned by the build-up of frost on the evaporator), a bellows-typeswitch is actuated by the condensation of fluid in the capillary tube.This bellows switch deenergizes the compressor until the capillary tubewarms to its abovementioned predetermined temperature and the bellowsswitch is closed by the expansion ofthc fluid in the capillary tube.While the compressor is off, the air circulating fan continues to drawroom air over the evaporator and to thus melt any frost build-upthereon.

Generally, capillary-tube sensors work well in most situations, however,these capillary tubes are expensive and may be easily displaced relativeto the evaporator during repair or cleaning of the air conditioner.upstream and down stream If the capillary tube contacts the evaporator,the compressor will run only a few seconds before the fluid in thecapillary tube chills below its condensation temperature and cuts offthe compressor. This is referred to as short cycling and. in essence.makes the air conditioner ineffective. On the other hand, if thecapillary tube is moved away from the evaporator, it may not sense thebuild-up of frost.

In US. Pat. No. 3,640,087 a bimetal thermostat is used to control boththe temperature of the room and to initiate a defrost cycle of the unit.However, before this bimetal thermostat can sense the temperature of theroom air, the room air is partially chilled by the evaporator so thatthe thermostat only indirectly senses room temperature. If this systemis subjected to drastic changes in ambient relative humidity andtemperature, or if the air conditioners filter becomes partiallyblocked, the temperature and flow rate of the air passing over thethermostat may be affected so that the room temperature cannotaccurately be controlled.

In refrigerators, freezers and other refrigerated units equipped with anautomatic defrost system, the defrost system is typically controlled bya timer which initiates operation of the defrost system at certain timesof the day or after the compressor has run a upstream and downstreampredetermined length of time. The rate at which frost builds up on theevaporator is a function of the amount of water vapor in the air passingover the evaporator, the greater the water vapor content the faster thefrost accumulates. For example, in a refrigerator the amount of watervapor in the air to be cooled depends on the ambient conditions (i.e.,room temperature and relative humidity) outside the refrigerator due toambient air being introduced into the refrigerator each time the door isopened, the frequency with which the door is opened, and water vaporsources (e.g.,' wet produce and open containers of liquids) within therefrigerator. With time-controlled defrost cycle initiation systems andwith a slow build up of frost on the evaporator, operation of thedefrost system is sometimes initiated before any significant amount offrost has built-up on the evaporator, thus resulting in a wastage ofpower to defrost the refrigerator when it is not required and exposingthe items in the refrigerator to unnecessary defrost cycles whichaccelerates spoilage. On the other hand, under heavy frost conditions,excessive frost may build upon the evaporator between the timed defrostcycles, thus reducing the efficiency of the refrigerator and increasingthe power consumed thereby.

UM ARY of THEJNYE IIQN Among the several objects of this invention maybe noted the provision of an automatic defrost cycle initiation systemfor refrigeration apparatus (e. g., an air conditioner, a refrigerator,a freezer, or other refrigerated unit) which initiates a defrost cycleupon sensing a build-up of frost on the evaporator of the refrigerationapparatus; the provision of such a defrost initiation system whicheliminates unneeded defrosting cycles which are inherent in theoperation of conventional defrost systems designed to handle worst casefrost conditions;

the provision of such a defrost cycle initiation system which conservespower and increases the operating efficiency of the refrigerationapparatus by eliminating unnecessary defrost cycles and by keeping therefrigeration apparatus free of excessive frost; the provision of such adefrost cycle initiation system which uses only thermal sensing elementsto initiate a defrost cycle; the provision of such a defrost initiationsystem in which the means for sensing the build-up of frost on theevaporator is a bimetal thermostat which may also be used to control thetemperature of the zone to be refrigerated or cooled by therefrigeration apparatus; the provision of such a defrost cycleinitiation system in which the bimetal thermostat is protected so as toprevent electric shock to persons cleaning or repairing therefrigeration apparatus; the provision of such a defrost initiationsystem which when applied to a room air conditioner prevents shortcycling (i.e., relatively short periods of compressor run time) whichmay be caused by erroneously sensing that frost has accumulated on theevaporator or that the room has been cooled to a desired temperature;the provision of such a defrost initiation system which will permit therefrigeration apparatus to run continuously when free of frost therebyto provide maximum cooling efficiency; and the provision of such adefrost cycle initiation system which is relatively simple, which is ofreliable operation, which is of economical construction and which willreliably operate regardless of the ambient climatic conditions. Otherobjects and features will be in part apparent and in part pointed outhereinafter.

Briefly, a defrost cycle initiation system of this invention is intendedfor use on refrigeration apparatus having cooling means for absorbingheat from a refrigerated zone, the cooling means being subject to frostbuild-up. The refrigeration apparatus has a main flow path for theintake of air from the refrigerated zone and for passage of the air overthe cooling means to chill the air and for the discharge of the chilledair into the refrigerated zone, and a blower for forcing air through themain flow path. The defrost cycle initiation system comprises anauxiliary flow path providing communication between the main flow pathand the refrigerated zone, air normally flowing in the auxiliary flowpath from the refrigerated zone to the main flow path when therefrigeration apparatus is in its normal mode of operation. Means isprovided for sensing the temperature of air flowing through theauxiliary flow path, this means initiating a defrost cycle upon sensinga predetermined initiation temperature of air. Means is also providedfor effecting a reversal of the flow of air in the auxiliary path inresponse to build-up of frost on the cooling means thereby to direct airof a temperature less than the predetermined depressed temperaturethrough the auxiliary flow path to lower the temperature of the airsensed by the sensing means below the predetermined initiationtemperature and for reestablishing the flow of air in the auxiliary flowpath from the refrigerated zone to the main flow path upon clearing thecooling means of frost.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a semi-diagrammatic view ofrefrigeration apparatus (e.g., a portion of a refrigerator-freezer unit)incorporating a first embodiment of an automatic defrost cycleinitiation system of this invention with arrows indicating the directionof the flow of air through the refrigeration apparatus and through thesystem of this invention when the evaporator is substantially free offrost build-up;

FIG. 2 is a view similar to FIG. 1 illustrating the flow of air throughthe refrigeration apparatus and through the system of this inventionwhen the evaporation is at least partially blocked by frost;

FIG. 3 is a partial perspective of a refrigeratorfreezer with its frontdoors removed and with a horizontal partition dividing it into a foodcompartment and a freezer compartment, a second embodiment of thedefrost cycle initiation system of this invention being incorporated inthe partition;

FIG. 4 is a plan view of the partition shown in FIG. 3 furtherillustrating the defrost cycle initiation system of this invention withthe arrows indicating the flow of air through the partition when theevaporator of the refrigerator-freezer is substantially free of frost;

FIG. 5 is a vertical section taken on line 5-5 of FIG.

FIG. 6 is a horizontal section taken on line 6-6 of FIG. 5;

FIG. 7 is a vertical sectional view taken on line 77 of FIG. 4;

FIG. 8 is a view similar to FIG. 4 illustrating the flow of air throughthe partition when the evaporator is at least partially blocked by frostbuild-up;

FIG. 9 is a vertical sectional view of a refrigeratorfreezerillustrating still another embodiment of the defrost cycle initiationsystem of this invention incorporated in the partition;

FIG. 10 is a first schematic of an electrical circuit for controllingthe refrigeration apparatus and the defrost cycle initiation system ofthis invention; I

FIG. 11 is a schematic of an alternative electrical circuit forcontrolling the refrigeration apparatus and the defrost cycle initiationsystem of this invention;

FIG. 12 is a perspective view of a room air conditioner unit with itscover removed, this room air conditioner incorporating a defrost cycleinitiation system of this invention;

FIG. 13 is a plan view of the room air conditioner illustrated in FIG.12 with parts borken away to further illustrate various refrigerationcomponents and the defrost cycle initiation system of this invention;

FIG. 14 is an enlarged view of a portion of FIG. 13 illustrating theflow of air (as indicated by the arrows) through the evaporator of theroom air conditioner and the flow of air through the defrost initiationsystem of this invention when the evaporator is substantially free offrost; and

FIG. 15 is a view similar to FIG. 14 illustrating the flow of airthrough the evaporator and through the defrost cycle initiation systemwhen the evaporator is at least partially blocked by frost build-upthereon.

Corresponding reference characters indicate corresponding partsthroughout the several views of the drawings.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the drawings andparticularly to FIGS. 1 and 2, a defrost cycle initiation system 1 forrefrigeration apparatus 3, for example, a refrigeratorfreezer 5a (asshown in FIGS. 1 and 2), 5b (as shown in FIGS. 3-8) or (as shown in FIG.9), or a room air conditioner 7 (shown in FIGS. 12-15), having coolingmeans, as indicated generally at 9, for absorbing heat from arefrigerated zone 11. The cooling means is subject to the build-up offrost F thereon. The refrigeration apparatus (whether it be arefrigerator-freezer or an air conditioner) has a main flow path 13 forthe intake of air from the refrigerated zone, for the passage of the air.over the cooling means to chill the air, and for the discharge of thechilled air into the refrigerated zone. A blower or fan is provided forforcing the air through the main flow path.

The defrost cycle initiation system 1 of this invention includes anauxiliary flow path 170 providing communication between main flow path13 and refrigerated zone 11. With cooling means 9 substantially free offrost build-up and with the refrigeration apparatus in its normal (orcooling) mode of operation, air flows in the auxiliary flow path fromthe refrigerated zone to the main flow path. Means 19, such as athermostatic switch, for sensing air temperature is provided within theauxiliary flow path to sense the temperature of air within the auxiliaryflow path and to initiate a defrost cycle upon sensing a predeterminedinitiation temperature of air flowing therethrough. The pressures withinthe main and the auxiliary flow path is such that it constitutes meansfor effecting a reversal of flow of air through the auxiliary flow path(see FIG. 2) in response to build-up of frost F on cooling means 9thereby to direct air having a temperature lower than the abovementionedpredetermined initiation temperature through the auxiliary flow path tolower the temperature of the air sensed by means 19 below the initiationtemperature so as to cold-bias the sensing means. Upon cooling means 9becoming clear of frost, the pressures within the main and the auxiliaryflow path is such that the flow of air from refrigerated zone 11 to mainflow path 13 is once again reestablished. While FIGS. 1 and 2 illustratethe defrost cycle initiation system of this invention installed in arefrigerator-freezer, it will be understood that this system may, withsuitable modifications be applied to a variety of refrigerated units andevaporator locations.

More particularly, a refrigerator-freezer 5b, as shown in FIGS. 3-8,having a defrost cycle initiation system 1 of this invention installedtherein is shown to have a cabinet 21 including a top wall 23, sidewalls 25, a back wall 27 and a horizontal partition 29 dividing theinterior of the cabinet into a freezer compartment 31 and a foodcompartment 33, these compartments constituting refrigerated zones 11.It will be understood that the fronts of the food and freezercompartments are normally closed by doors (not shown). In operation, thefreezer compartment is maintained at a temperature (e.g., 0 F.) wellbelow the maximum temperature of the, food compartment (e.g., 40-42 F.)The refrigeratorincludes a refrigeration system including a compressorand motor 35 (see FIGS. 14) and 11), a condenser (not shown), anevaporator 37 constituting cooling means 9, and various refrigerantlines (not shown) interconnecting the various refrigeration components.The evaporator is shown mounted within a channel 39 (see FIG. 5) inpartition 29, this channel constituting main flow path 13 and beingdefined by a lower panel 411 constitutingthe top of the "foodcompartment and an upper panel 43 spaced above lower panel 11 andconstituting the floor of the freezer compartment. Lower panel 41 has aseriesofopenings 45 therein adjacent its front edge and upper panel 43has a series of openings 47 therein generally above openings 45, theseopenings constituting inlet openings for the intake of air (return air)into the main flow path 13 from both the food and freezer compartments31 and 33. This return air is drawn by blower 15 over evaporator 37which absorbs heat from the return air and chills it. The blower thenforces the chilled air to the back of the cabinet and discharges it intothe freezer and food compartments via vents 49 in the freezercompartment and outlet 51 in the food compartment (see FIGS. 7 and 9). Adefrost heater H 1 is provided in the channel 39 below evaporator 37 toheat up the evaporator so as to clear the evaporator of frost uponinitiation of a defrost cycle, the heater being energized anddeenergized in response to the defrost cycle initiation system 1 of thisinvention. Evaporator 37 may, for example, be a finned refrigerant lineformed to constitute a helical coil. Heater H1 may either be a Radiantheater, such as shown, or a conduction heating element carried on theexterior of the evaporator coil for melting frost from the evaporator byconduction. It will be noted that the return air in main flow path 13upstream of evaporator 37 is a mixed air flow from both the freezer andfood compartments 31 and 33, respectively, and the temperature of thismixed flow of air (e.g., +8 F.) is substantially below the temperatureof the return air in the food compartment (e.g., 35 F. to 42 F.) andbelow the initiation temperature (e.g., 23 F.)

In FIGS. 3-8, auxiliary flow path 17b, is shown to'be formed inpartition 29 at one side (the left side as viewed in FIG. 3) of channel39. The auxiliary flow path includes an auxiliary inlet 53 in partition29 in communication with food compartment 33 for the intake of air fromthe food compartment, and an inletoutlet opening 55 also in partition 29in communication with main flow path 13 on the upstream side ofevaporator 37. A duct 57 is formed in partition 29 and providescommunication between auxiliary inlet 53 inlet/outlet opening 55, and anoutlet 59 downstream of the evaporator in main flow path 13. A defrostinitiation thermostat TSla is mounted in the auxiliary flow pathadjacent auxiliary inlet 53 in inlet/outlet opening 55, this defrostinitiation thermostat constituting means 19 for sensing the temperatureof the air flowing through the auxiliary flow path and initiating adefrost cycle (in a manner as will appear) when the temperature of theair in the auxiliary flow path drops below the above-mentionedpredetermined initiation temperature.

More particularly, thermostat TSla is a normal differential, bimetalthermostat. A temperature control thermostat (cold control) TS3a ispositioned in food compartment 33 in a selected location (see FIG. 7) tosense and to regulate the temperature of the air in the foodcompartment. A defrost termination thermostat T52 is mounted withinchannel 39 adjacent heater H1. These components are electricallyinterconnected as illustrated in FIG. 10.

Referring now to FIG. 10, compressor motor 35 is connected in a seriescircuit across electrical power supply lines L1 and L2, this seriescircuit including defrost initiation thermostat TS 1a and cold controlthermostat TS3a. Fan 15 is shunt-connected across compressor motor 35 sothat these two units are concurrently energized in response to thecontacts of thermostats TSla and TS3a being in their positions as shownin FIG. 10. The defrost initiation thermostat TSla, when sufficientlycooled (i.e., when cooled to its predetermined initiation temperaturewhich, for example, may be approximately 23 F.), is actuated to thealternate position of its contacts to disconnect the fan and compressorand to supply power (when thermostat TS3a is closed) to defrost heaterH1 and to defrost termination thermostat TS2. A booster heater Rla(e.g., a small resistor-type heater) is optionally enclosed withinthermostat TSla for reheating the thermostat for reset purposes, thisbooster heater being shuntconnected across the contacts of thermostatTSla for resetting the contacts thereof by heating the thermostaticelement therein after the thermostat has been cold-biased below itspredetermined initiation temperature. Both thermostatic switches T510and T82 are of the wide differential type, thermostat TSla beingactuated from its FIG. 10 position to complete the defrost heater H1circuit in response to sensing its predetermined depressed temperatureand remaining in that position until thermostat TSla warms to asubstantially higher temperature. The switch contacts of defrosttermination thermostat TS2 will remain closed, as shown in FIG. 10,until the temperature within channel 39 rises to about 85 F., thesecontacts remaining open until thermostat TS2 recools to a substantiallylower temperature upon restart of compressor 35 (e.g.,

With refrigerator 5b operating in its normal refrigeration mode, thecontacts of cold control thermostat TS3a and those of defrosttermination thermostat TS2 are closed and the contacts of defrostinitiation thermostat TSla are positioned as shown in FIG. so as toenergize compressor motor 35 and blower and to maintain heater H1deenergized. With blower 15 operating and with evaporator 37substantially free of frost F, the blower draws in air from therefrigerated zones (e.g., from the freezer and food compartments 31 and33, respectively) into main flow path 13 (i.e., channel 39) by means ofopenings 45 and 47. This air passes over evaporator 37 which absorbsheat from the air and chills the air. Upon contact with the evaporator,excess water vapor in the air condenses on the evaporator and formsfrost F. The rate at which frost builds up on the evaporator dependsupon the moisture content in the air to be chilled and the relativelength of the on cycle of compressor unit 35.

With evaporator 37 substantially free of frost F, the pressure ontheupstream side of the evaporator is substantially the same as thepressure on the downstream side of the evaporator, and consequentlyblower 15 draws air from the food compartment 33 via auxiliary inlet 53and discharges this relatively warm air (e.g., 35 42 F.) into main flowpath 13 via inlet/outlet opening 55 on the upstream side of theevaporator and outlet opening 59 on the downstream side of theevaporator. This relatively warm air from the food compartment in theauxiliary flow path passes over defrost initiation thermostat TSla andmaintains the contacts thereof as shown in FIG. 10. Upon frost buildingup on the evaporator to such a degree that the flow of air therethroughis at least partially blocked or restricted, the pressure in the mainflow path on the downstream side of the evaporator drops below thepressure on the upstream side of the evaporator and thus effects apressure differential across the evaporator. This pressure differentialcauses a reversal of the flow of air in auxiliary flow path 17b so thatair from main flow path 13 on the upstream side of evaporator 37 entersthe auxiliary flow path via inlet/outlet opening 55 and flows overthermostat TSla. This air from the main flow path is colder (e.g., 8 F.)than the predetermined initiation temperature (e.g., 23 F.) ofthermostat TSla and thus cold-biases thermostat TSla so as to move itscontacts from the position shown in FIG. 10 and to connect defrostheater H1 to lines L1 and L2. With the contacts of cold controlthermostat TS3a and defrost termination thermostat TS2 closed, defrostheater H1 is energized and thus heats evaporator 37 so as to clear it offrost. Upon the temperature within channel 39 rising to a predeterminedelevated temperature (e.g., F.), the contacts of termination thermostatTS2 open and deenergize defrost heater H1. Defrost heater H1 may, forexample, be in its defrost mode for about 10 minutes. Booster heater Rlais shunt-connected across the contacts of thermostat TSla and isenergized whenever the contacts of cold control thermostat TS3a areclosed. Thus, heater Rla is energized concurrently with the energizationof defrost heater H1 so that the heat from the defrost heater and theheat generated by the booster heater reset the contacts of thermostatTSla. Normally, the time required to reset the contacts of thermostatTSla is appreciably longer than the time required for terminationthermostat TS2 to deenergize defrost heater Hl. For example, it mayrequire 20-25 minutes to reset the contacts of thermostat TSla while itmay require only 11 minutes for thermostat TS2 to deenergize heater Hl.This time differential (e.g., 9-14 minutes) is referred to as the dripor soak timeduring which the temperature within channel 39 remainselevated so as to insure that all frost and ice within channel 39 ismelted and that any water may drain from the channel. Upon resetting ofthe contacts of the defrost initiation thermostat TSla and upon thetemperature of the food compartment 33 risingto a preestablishedtemperature, cold control TS3a is actuated thereby to energizecompressor motor 35 and fan 15 to again place the refrigerator in itsnormal mode of operation. It will be noted in certain applicationstermination thermostat TS2 in FIG. 10 may be replaced by a thermal fuse.

In FIG. 9, illustrating a third embodiment 5c of a refrigerator-freezerin which auxiliary flow path is formed in partition 29 below channel 39thereby to show that the routing and configuration of the auxiliary flowpath may be varied, but so long as it interconnects refrigerated zone 11and main flow path 13 on both the upstream and downstream sides ofevaporator 37, the reversal of air in the auxiliary flow path will beeffected by the build-up of frost on the evaporator. FIG. 9 depicts acold control thermostat TS3b located in a selected location in foodcompartment 33 and a defrost heater initiation thermostat TSlb (alsoreferred to as a thermostatic switch) located in auxiliary flow path17c. Initiation thermostat TSlb has an optional booster heater Rlbenclosed therewithin, this booster heater heating the contacts of thethermostat for reset purposes in a preselected time. It will be notedthat thermostat TSlb may be reset in a preselected time by the airwithin auxiliary flow path 17c which has been heated by heater H1. InFIG. 11, booster R112 is shown to be shunt-connected across heater H1for concurrent energization therewith, operation of the circuit shown inFIG. 11 will be more particularly described hereinafter.

Referring now to FIG. 11, compressor motor 35 is shown connected in aseries circuit somewhat different from the circuit shown in FIG. 10,this new circuit including the normally open contacts of defrost cycleinitiation thermostat TSlb and the contacts of temperature controlthermostat (cold control) T8312 across electrical power supply lines LI.and L2. Blower I is shunt-connected across the compressor motor 35 sothat these two units are concurrently energized and deenergized. Coldcontrol thermostat TS3b is positioned within food compartment 33 in aselected location (see FIG. 9) to sense and to regulate the temperatureof the air in food compartment 33. The defrost initiation thermostatTSlb, when cooled (i.e., cold-biased) below its initiation temperature(e.g., 23 F.) is actuated to close its contacts and to connect heater Hito line L2. Subsequent to thermostat TSlb closing and upon cold controlthermostat TS3b sensing that the food compartment has attained a desiredlower temperature, the contacts of thermostat TS3b are reversed from theposition shown in FIG. 11 so as to disconnect the compressor fan and toconnect heater H]. to line Ll thereby initiating the defrost cycle.Booster heater Rlb is shunt-connected across defrost heater HI andenergized concurrently with the defrost heater.

Operation of the defrost cycle initiation system I of this invention,wired in accordance with FIG. 11, as included in a refrigerator-freezer5c, is as follows:

With refrigerator 50 operating in its normal refrigeration mode, thecontacts of cold control thermostat TS3b are closed and those of defrostinitiation thermostat TSlb are open, as shown in FIG. 11, therebyenergizing compressor motor 35 and blower l5 and maintaining heater Hlldeenergized. With blower l5 operating and with evaporator 37substantially free of frost, the blower draws in air from therefrigerated zones 11 (e.g., from freezer compartment 31 and from foodcompartment 33) via inlets 45 and 47 into channel 39. This air passesover evaporator 37 which absorbs heat from the air and chills the air.

With evaporator 37, substantially free of frost, the pressure in channel39 on the upstream side of the evaporator is substantially the same asthe pressure on the downstream side of the evaporator and consequentlyblower l5 draws air from food compartment 33 via auxiliary inlet 53 andthen discharges this relatively warm air into channel 39 viainlet/outlet opening 55 and outlet 59 on both the upstream anddownstream sides of the evaporator. This relatively warm air passes overdefrost initiation thermostat TSlb in auxiliary flow path 170 andmaintains the contacts thereof open. Upon frost build-up on theevaporator to such a degree that the flow of air over the evaporator isat least partially blocked or restricted, the pressure in the main flowpath on the downstream side of the evaporator is caused to drop belowthe pressure on the upstream side of the evaporator thereby to create apressure differential across the evaporator. This pressure differentialcauses a reversal of the flow of air in the auxiliary flow path so thatair from the main flow path on the upstream side of the evaporatorenters the auxiliary flow 7 path via inlet/outlet opening 55 and flowsover thermomostat TS3b sensing that the food compartment has beenchilled to a desired temperatureteg, 35 F), it deenergizes compressormotor 35 and blower and completes the circuit to heater H1 thereby toinitiate the defrost cycle. Booster heater Rlb is energized concurrentlywith heater H l. Booster heater Rlb requires approximately 1 1 minutesto heat thermostat TSlb to a temperature sufficient to reset (open) itscontacts and to deenergize heater H1. Thus heater H1 remains energizedfor about I 1 minutes. During the time heater H1 is energized and forsome time thereafter, the temperature in food compartment 33 rises to apredetermined temperature (e.g., 40-42 F.) to reset the contacts of coldcontrol TS3b. Typically, cold control TS3b will reset in about 25minutes. This difference be- 7 tween the reset times of the cold controlthermostat stat TSlb. This air from the main flow path is colder (e.g.,8 F.) than the predetermined initiation temperature (e.g., 23 F.) ofthermostat TSlb and thus coldbiasesthermostat TSlb so as to close itscontacts and connect heater H1 to line L2. Upon cold control therand thedefrost initiation thermostat (e.g., 14 minutes) is referred to as thesoak or drip time during which the temperature within channel 39 remainselevated to complete defrosting of the frost on the evaporator, to

melt any ice which may have accumulated in the main flow path and topermit any water to drain from channel 39. Thus, it can be seen that thecircuits shown in FIGS. 10 and 11 essentially serve the same function,but the thermostat TSlb in FIG. 11 serves a dual function of defrostcycle initiation and defrost heater termination so that the defrostheater termination thermostat in FIG. 10 may be omitted.

It will be particularly noted that the defrost cycle initiation system 1of this invention, as applied to a combination refrigerator-freezer 5a,5b or 50 initiates a defrost cycle only in response to a build-up of apredetermined amount of frost on evaporator 37, and accordingly thefrequency of defrost cycles and the exposure of refrigerated or frozenitems to unnecessary defrost cycles is minimized. Also, the powerconsumed by a refrigerator-freezer incorporating the demand defrostcycle initiation system of this invention is reduced by elimination ofunnecessary defrost cycles and by keeping the evaporator free ofexcessive frost build-up, thereby to maintain a high heat transferefficiency of the evaporator. Use of the defrost cycle initiation systemof this invention also eliminates the use of timers to initiate thedefrost cycle.

Referring to FIGS. 12 -15 which illustrate the defrost cycle initiationsystem 1 of this invention as installed in a room air conditioner unit(RAC) 7, the RAC is shown in FIG. 12 with its cover removed. The RACincludes a base 101 and a front face plate 103 having an air inlet grill105 through which room air is inducted into main flow path 13 forpassage over cooling means 9, an air outlet grill 107 for directingchilled air back into the room, and an escutcheon plate 109 mounting themanual controls for the RAC. In FIG. 13, the refrigeration system forthe RAC is shown to include a compressormotor unit 111, a condenser 113,a condenser fan for blowing outside air over the condenser, and anevaporator 117, this evaporator constituting cooling means 9. It will beunderstood that refrigerant lines interconnect the compressor, condenserand evaporator in the conventional manner so that the compressor maycirculate refrigerant through these components for cooling purposes. Theevaporator is separated from the condenser by means of a wall orpartition 119 so that air cooled by the evaporator is not mixed with therelatively warm outside air circulated by the condenser fan. Theevaporator is a conventional expansion coil refrigerant line having fins121 and end turns 123 for the various turns of the coil. An air filter125 is shown positioned behind inlet grill 105, but it will beunderstood that the filter may be placed immediately in front ofevaporator 117. A centrifugal blower 127 is provided behind evaporator117 to draw room air into the room air conditioner via grill 105, todraw this room air over the evaporator to be cooled and to exhaust thecooled air back into the room (i.e., the refrigerated zone 11) viaoutlet grill 107. A motor 128 drives both condenser fan 115 andevaporator blower 127. A horizontal panel 129 constituting an airdivider is installed behind the evaporator to isolate the blower inletfrom the blower outlet. This air divider has an opening 131 constitutingan outlet for blower 127 and provides a passage for the chilled air tobe directed to outlet grill 107. It will be understood that inlet grill105, base 101, the cover (not shown), partition 119, air divider 129,and outlet grill 107 together constitute main flow path 13.

End turns 123 at one end of evaporator 117 (i.e., the left end as viewedin FIG. 13) are enclosed in an endturn enclosure or compartment 133. Athermostat enclosure 135 is provided adjacent end-turn enclosure 133 anda bimetal thermostat 137 is mounted within the thermostat enclosure,this thermostat constituting temperature sensing means 19. Thermostat137 is a relatively narrow differential (e.g., having a differential of3 6 F adjustable thermostat. It will be noted that with thermostat 137installed in enclosure 135, the thermostat is protected so as to preventelectrical shock during ordinary use and maintenance of the RAC. Anauxiliary air inlet 139 is provided in front face plate 103, thisauxiliary inlet opening being in communication with thermostat enclosure135. As shown in FIGS. 1315, end-turn enclosure 133 has an opening 141providing communication between main flow path 13 on the upstream sideof evaporator 117 and the endturn enclosure, and an opening 143providing communication between the end-turn enclosure and thethermostat enclosure, the latter having an opening 145 providingcommunication between enclosure 135 and the main flow path on thedownstream side of the evaporator. Thus, auxiliary inlet 139, enclosures133 and 135, and openings 141 and 143 and 145 together constituteauxiliary air flow path 17d.

Operarion of the defrost initiation system of this invention asinstalled on a room air conditioner will be further describedhereinafter. With evaporator 117 substantially free of frost build-up,blower 127 draws room air into main flow path 13 via inlet grill 105 andinto auxiliary flow path 17d via auxiliary inlet 139. This room airpasses over thermostat 137 thereby to permit the thermostat toaccurately sense the temperature of the room air (thus permitting thethermostat to accurately regulate the room temperature) and exitsenclosure 135 via openings 143 and 145. A portion of the room air in theauxiliary flow path enters end-turn compartment 133 and is drawn intomain flow path 13 on the upstream side of evaporator 117. Room airpassing through the evaporator and being cooled thereby causes watervapor to condense on the evaporator. Since the temperature of theevaporator may be below 32 F frost F may begin to build-up on theevaporator. Upon a quantity of frost accumulating on evaporator 117, theflow of room air therethrough is partially restricted thereby causing apressure differential across the evaporator. This difference in pressurecauses air from main flow path 13 on the upstream side of evaporator 117to enter enclosures 133 and 135 via openings 141 and 143, respectively.As this air passes through the end-turn compartment it is cooled byevaporator end-turns 123 to a temperature well below the roomtemperature (e.g., below F.) which cold biases thermostat 137 and thusstops compressor 111. Blower 127 continues to blow relatively warm roomair (e.g., 72 F.) over the evaporator to thus clear it of frost. Uponthe air flowing through auxiliary flow path 17 via end-turn compartment133 warming above the abovementioned compressor start temperature (e.g.72-75 F) thermostat 137 again energizes the compressor and the RAC maybe again operated in its normal mode free of frost F.

Thermostat 137 is wired in a series circuit (not shown) with the motorof compressor unit 111 so that the compressor is energized anddeenergized by the closing and opening of the thermostat contacts. Forexample, these contacts may open to terminate operation of thecompressor when the thermostat senses that the air temperature inauxiliary flow path 17 is about 68 71 F. and may close to initiateoperation of the compressor when the thermostat senses that the airtemperature in the auxiliary flow path rises to about 72 75 F. Blowermotor 128 is preferably continuously energized so that blower 127 drawsroom air over evaporator 117 at all times, regardless of the operationof compressor unit 111. When the compressor unit is deenergized, theflow of room air (e.g., 72 F. air) over the evaporator clears theevaporator of any frost F which may have built up thereon. In some RACunits, motor 128 may be energized concurrently with the compressor, butit will be understood that the defrost initiation system of thisinvention will function regardless of whether the evaporator bloweroperates continuously or intermittently.

In accordance with this invention, sensing means 19 (especially in aroom air conditioning application) may serve the dual function of a roomtemperature regulator (cold control) and a defrost cycle initiationsystem. As seen in FIGS. 11 and 12, when evaporator 117 is free offrost, room air in sufficient quantity is drawn into thermostatenclosure 135 so that thermostat 137 may directly and accurately sensethe temperature of the room air thereby to provide accurate control ofthe room temperature by terminating operation of the compressor when thethermostat senses a first predetermined temperature (e.g., 68-7 1 F.)and by starting operation of the compressor when the thermostat senses asecond predetermined temperature (e.g., 7275 F. When the evaporatorbecomes at least partially blocked by frost F (see FIG. 15), a reversalof the flow of air from thermostat enclosure 135 and from end-turnenclosure or compartment 133 is effected so that air from main flow path13 on the upstream side of the evaporator 117 flows into the end-turnenclosure thereby to be chilled by end turns 123 well below the roomtemperature (e.g., less than 60 F.) so as to coldbias thermostat 137 andto thus terminate operation of compressor unit 111. Blower 131 willcontinue to draw in relatively warm room air and to pass it over theevaporator thereby to melt any accumulation of frost thereon.

It will be understood that if filter becomes clogged with dirt therebyrestricting the flow of air through main flow path 13, the defrost cycleinitiation system of this invention will continue to operatesatisfactorily but its sensitivity may be affected. It will further beunderstood that the defrost cycle initiation system of this inventionmay be operated satisfactorily if the outlet 145 from thermostatenclosure 135 is in communication with a low pressure region behindpartition 119 so as to place enclosure 135 in communica tion withcondenser fan 115 rather than with evaporator blower E27,.

In view of the above, it will be seen that the several objects of theinvention are achieved and other advantageous results attained.

As various changes could be made in the above constructions withoutdeparting from the scope of the invention, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

What is claimed is:

l. A defrost cycle initiation system for refrigeration apparatus havingcooling means for absorbing heat from a refrigerated zone, said coolingmeans being subject to frost build-up, said refrigeration apparatushaving a main flow path for the intake of air from said refrigeratedzone, for passage of the air over said cooling means to chill the airand for the discharge of the chilled air into the refrigerated zone, anda blower for forcing the air through said main flow path; said systemcomprising an auxiliary flowpath providing communication between saidmain flow path and said refrigerated zone, air normally flowing, in saidauxiliary flow path from said refrigerated zone to said main flow pathwhen said refrigeration apparatus is in its normal mode of operation,means for sensing the temperature of air flowing through said auxiliaryflow path adapted to initiate a defrost cycle upon sensing apredetermined initiation temperature of air, and means for effecting areversal of flow of air in said auxiliary path in response to build-upof frost on said cooling means there-by to direct air having atemperature less than said predetermined initiation temperature throughsaid auxiliary flow path to lower the temperature of the air sensed bythe sensing means below said predetermined initiation temperature andfor reestablishing the flow of air in said auxiliary flow path from saidrefrigerated zone to said main flow path upon clearing the cooling meansof frost.

2. A defrost cycle initiation system as set forth in claim 1 whereinsaid refrigeration apparatus is a refrigerated unit, such as arefrigerator or the like, said refrigerated zone is a refrigeratedcompartment and said cooling means is an evaporator, said main flow pathbeing in communication with said refrigerated zone and said evaporatorbeing located in said main flow path so that air from said refrigeratedcompartment enters said main flow path, passes over said evaporator forbeing chilled, and is discharged into the refrigerated compartment, saidauxiliary flow path being in communication with the refrigeratedcompartment for the intake of air from the refrigerated compartment andfor the discharge of compartment air into the main flow path on theupstream and downstream side of the evaporator when the evaporator issubstantially free of frost, said auxiliary flow path providingcommunication between the main flow path on the upstream side of theevaporator and the main flow path on the downstream side of theevaporator for the intake of air from the main flow path-on the upstreamside of the evaporator and for the discharge of said air from the mainflow path back into the main flow path on the downstream side of theevaporator when the evaporator is at least partially blocked by frost,the temperature of said air from the main flow pathon the upstream sideof the evaporator being belowsaid predetermined initiation temperaturewhen said evaporator is at least partially blocked by frost, thereby toactuate said sensing means upon said air from the upstream side of saidevaporator flowing through the auxiliary flow path and to initiate adefrost cycle.

3. A defrost cycle initiation system as set forth in claim 2 whereinsaid refrigerated unit is a refrigeratorfreezer having a foodcompartment and a freezer compartment, said food and freezercompartments constituting said refrigerated zone, said freezercompartment being maintained at a. colder temperature than said foodcompartment, said main flow path being in communication with the foodcompartment and the freezer compartment for the intake of air from bothsaid compartments, said auxiliary flow path being in communication withsaid food compartment so that with the evaporator substantially free offrost said blower means draws in relatively warm air from said foodcompartment into the auxiliary flow path for discharge of this foodcompartment air into the main jflow path on the upstream and downstreamside of the evaporator, and upon build-up of frost on the evaporatorsaid blower draws air from the mainflow path on the upstream side of theevaporator into said auxiliary flow path for flowing over said sensingmeans thereby to cold-bias the latter below said predeterminedinitiation temperature and initiate a defrost cycle.

4,. A defrost cycle initiation system as set forth in claim 3 whereinsaid refrigerator-freezer has a defrost heater located adjacent saidevaporator, said heater, upon energization thereof, clearing saidevaporator of frost, said heater being energized by said sensing meansthereby to clear said evaporator of frost.

5. A defrost cycle initiation system as set forth in claim 4 whereinsaid refrigerator-freezer has a compressor and a cold control thermostatfor energization and deenergization of said compressor there-by toregulate the temperature of the refrigerated zone, said sensing meanscomprising a thermostatic switch, said cold control thermostat and saidthermostatic switch adapted to energize said heater upon saidthermostatic switch sensing said predetermined initiation temperatureand said cold control thermostat sensing a desired temperature in saidfood compartment, said thermostatic switch adapted to reset in apreselected time to deenergize said heater.

6. A defrost cycle initiation system as set forth in claim 1 whereinsaid refrigeration apparatus is an air conditioner, said refrigeratedzone is a room which is to be cooled, and said cooling means is anevaporator, said auxiliary flow path being in communication with theroom for the intake of room air and for the discharge of this room airinto the main flow path on the upstream and downstream side of theevaporator when the evaporator is substantially free of frost build-upand for the intake of air from said main flow path on the up stream sideof the evaporator and for discharge of said air from the main flow pathback into the main flow path on the downstream of the evaporator whenthe evaporator is at least partially blocked by frost buildup, thetemperature of said air from the main flow path on the upstream side ofthe evaporator being cooled by the at least partially blocked evaporatorto a temperature below room temperature whereby said flow of air fromthe upstream side of the evaporator through said auxiliary flow pathactuating said sensing means and initiating said defrost cycle.

7. A defrost cycle initiation system as set forth in claim 6 whereinsaid air conditioner has a main inlet for intake of room air into saidmain flow path and an auxiliary air inlet for intake of room air intothe auxiliary flow path, an enclosure housing said sensing means, saidenclosure constituting a portion of said auxiliary flow path, a firstopening in said enclosure providing communication between said housingand said main flow path on the upstream side of said evaporator and asecond opening in said enclosure in communication with said main flowpath on the downstream side of the evaporator, whereby with saidevaporator substantially free of frost, room air enters said enclosurevia said auxiliary inlet and at least part of said room air enteringsaid enclosure is discharged into said main flow path via said firstopening and whereby upon frost accumulating on said evaporator, air isdrawn into said enclosure from said main flow path via said firstopening, said air from said main flow path drawn into the first openingbeing chilled below said room temperature.

8. A defrost cycle initiation system as set forth in claim 7 whereinsaid first opening is adjacent a portion of said evaporator whereby uponsaid flow of air in said main flow path through the evaporator becomingat least partially restricted by the build-up of frost on theevaporator, said flow of air from the main flow path on the upstreamside of the evaporator into the enclosure via said first opening ischilled by said evaporator thereby to cold-bias said sensing means toinitiate a defrost cycle.

9. A defrost cycle initiation system as set forth in claim 8 whereinsaid evaporator is a coiled refrigerant line having a plurality of endturns, said apparatus including a compartment enclosing said end turnsof the evaporator, said end turn compartment being in communication withsaid main flow path on the upstream side of the evaporator and being incommunication with the enclosure via said first opening whereby withsaid evaporator substantially free of frost build-up at least part ofsaid air flows from said auxiliary air inlet into said main flow path onthe upstream side of the evaporator via said enclosure and said end turncompartment, and with said evaporator at least partially blocked byfrost, the flow of air in said auxiliary flow path is reversed so thatair from the main flow path on the upstream side of the evaporatorenters the end turn compartment thereby to be chilled by the evaporatorend turns and then enters said enclosure thereby to cold-bias saidsensing means and initiate a defrost cycle.

10. A defrost cycle system as set forth in claim 7 wherein said airconditioner includes a compressor and wherein said sensing means is abimetal thermostat, said thermostat being operable to initiate operationof said compressor upon sensing air at a first predetermined temperatureand to terminate operation of the compressor upon sensing air at asecond and lower predetermined temperature, whereby with said evaporatorsubstantially free of frost and with said thermostat sensing the roomair temperature said thermostat initiating operation of the compressorupon the temperature of the room air rising to said first predeterminedtemperature and said thermostat terminating operation of said compressorupon the temperature of the room air dropping to said secondpredetermined temperature.

11. A defrost cycle initiation system as set forth in claim 1 whereinsaid refrigeration apparatus includes a refrigeration system comprisinga compressor and an evaporator, the latter constituting said coolingmeans, said sensing means being operable to initiate operation of thecompressor upon sensing a first predetermined temperature and toterminate operation of the compressor upon sensing a second and lowerpredetermined temperature, whereby with said evaporator substantiallyclear of frost the temperature sensing means senses the temperature ofair from the refrigerated zone so that upon the temperature of the airin the refrigerated zone rising to said first predetermined temperaturesaid sensing means initiates operation of the compressor, and upon thetemperature of the air in the refrigerated zone falling to said secondpredetermined temperature said sensing means terminates operation of thecompressor and thus maintains the temperature of the air in therefrigerated zone between said compressor start and compressor stoptemperatures.

12. A defrost cycle initiation system as set forth in claim 11 whereinsaid refrigeration apparatus is a room air conditioner and saidtemperature sensing means is a bimetal thermostat.

13. A defrost cycle initiation system as set forth in claim 1 whereinsaid refrigeration apparatus is a room air conditioner and saidtemperature sensing means is a bimetal thermostat.

1. A defrost cycle initiation system for refrigeration apparatus havingcooling means for absorbing heat from a refrigerated zone, said coolingmeans being subject to frost build-up, said refrigeration apparatushaving a main flow path for the intake of air from said refrigeratedzone, for passage of the air over said cooling means to chill the airand for the discharge of the chilled air into the refrigerated zone, anda blower for forcing the air through said main flow path; said systemcomprising an auxiliary flow path providing communication between saidmain flow path and said refrigerated zone, air normally flowing in saidauxiliary flow path from said refrigerated zone to said main flow pathwhen said refrigeration apparatus is in its normal mode of operation,means for sensing the temperature of air flowing through said auxiliaryflow path adapted to initiate a defrost cycle upon sensing apredetermined initiation temperature of air, and means for effecting areversal of flow of air in said auxiliary path in response to build-upof frost on said cooling means there-by to direct air having atemperature less than said predetermined initiation temperature throughsaid auxiliary flow path to lower the temperature of the air sensed bythe sensing means below said predetermined initiation temperature andfor reestablishing the flow of air in said auxiliary flow path from saidrefrigerated zone to said main flow path upon clearing the cooling meansof frost.
 2. A defrost cycle initiation system as set forth in claim 1wherein said refrigeration apparatus is a refrigerated unit, such as arefrigerator or the like, said refrigerated zone is a refrigeratedcompartment and said cooling means is an evaporator, said main flow pathbeing in communication with said refrigerated zone and said evaporatorbeing located in said main flow path so that air from said refrigeratedcompartment enters said main flow path, passes over said evaporator forbeing chilled, and is discharged into the refrigerated compartment, saidauxiliary flow path being in communication with the refrigeratedcompartment for the intake of air from the refrigerated compartment andfor the discharge of compartment air into the main flow path on theupstream and downstream side of the evaporator when the evaporator issubstantially free of frost, said auxiliary flow path providingcommunication between the main flow path on the upstream side of theevaporator and the main flow path on the downstream side of theevaporator for the intake of air from the main flow path on the upstreamside of the evaporator and for the discharge of said air from the mainflow path back into the main flow path on the downstream side of theevaporator when the evaporator is at least partially blocked by frost,the temperature of said air from the main flow path on the upstream sideof the evaporator being below said predetermined initiation temperaturewhen said evaporator is at least partially blocked by frost, thereby toactuate said sensing means upon said air from the upstream side of saidevaporator flowing through the auxiliary flow path and to initiate adefrost cycle.
 3. A defrost cycle initiation system as set forth inclaim 2 wherein said refrigerated unit is a refrigerator-freezer havinga food compartment and a freezer compartment, said food and freezercompartments constituting said refrigerated zone, said freezercompartment being maintained at a colder temperature than said foodcompartment, said main flow path being in communication with the foodcompartment and the freezer compartment for the intake of air from bothsaid compartments, said auxiliary flow path being in communication withsaid food compartment so that with the evaporator substantially free offrost said blower means draws in relatively warm air from said foodcompartment into the auxiliary flow path for discharge of this foodcompartment air into the main flow path on the upstream and downstreamside of the evaporator, and upon build-up of frost on the evaporatorsaid blower draws air from the main flow path on the upstream side ofthe evaporator into said auxiliary flow path for flowing over saidsensing means thereby to cold-bias the latter below said predeterminedinitiation temperature and initiate a defrost cycle.
 4. A defrost cycleinitiation system as set forth in claim 3 wherein saidrefrigerator-freezer has a defrost heater located adjacent saidevaporator, said heater, upon energization thereof, clearing saidevaporator of frost, said heater being energized by said sensing meansthereby to clear said evaporator of frost.
 5. A defrost cycle initiationsystem as set forth in claim 4 wherein said refrigerator-freezer has acompressor and a cold control thermostat for energization anddeenergization of said compressor there-by to regulate the temperatureof the refrigerated zone, said sensing means comprising a thermostaticswitch, said cold control thermostat and said thermostatic switchadapted to energize said heater upon said thermostatic switch sensingsaid predetermined initiation temperature and said cold controlthermostat sensing a desired temperature in said food compartment, saidthermostatic switch adapted to reset in a preselected time to deenergizesaid heater.
 6. A defrost cycle initiation system as set forth in claim1 wherein said refrigeration apparatus is an air conditioner, saidrefrigerated zone is a room which is to be cooled, and said coolingmeans is an evaporator, said auxiliary flow path being in communicationwith the room for the intake of room air and for the discharge of thisroom air into the main flow path on the upstream and downstream side ofthe evaporator when the evaporator is substantially frEe of frostbuild-up and for the intake of air from said main flow path on theupstream side of the evaporator and for discharge of said air from themain flow path back into the main flow path on the downstream of theevaporator when the evaporator is at least partially blocked by frostbuild-up, the temperature of said air from the main flow path on theupstream side of the evaporator being cooled by the at least partiallyblocked evaporator to a temperature below room temperature whereby saidflow of air from the upstream side of the evaporator through saidauxiliary flow path actuating said sensing means and initiating saiddefrost cycle.
 7. A defrost cycle initiation system as set forth inclaim 6 wherein said air conditioner has a main inlet for intake of roomair into said main flow path and an auxiliary air inlet for intake ofroom air into the auxiliary flow path, an enclosure housing said sensingmeans, said enclosure constituting a portion of said auxiliary flowpath, a first opening in said enclosure providing communication betweensaid housing and said main flow path on the upstream side of saidevaporator and a second opening in said enclosure in communication withsaid main flow path on the downstream side of the evaporator, wherebywith said evaporator substantially free of frost, room air enters saidenclosure via said auxiliary inlet and at least part of said room airentering said enclosure is discharged into said main flow path via saidfirst opening and whereby upon frost accumulating on said evaporator,air is drawn into said enclosure from said main flow path via said firstopening, said air from said main flow path drawn into the first openingbeing chilled below said room temperature.
 8. A defrost cycle initiationsystem as set forth in claim 7 wherein said first opening is adjacent aportion of said evaporator whereby upon said flow of air in said mainflow path through the evaporator becoming at least partially restrictedby the build-up of frost on the evaporator, said flow of air from themain flow path on the upstream side of the evaporator into the enclosurevia said first opening is chilled by said evaporator thereby tocold-bias said sensing means to initiate a defrost cycle.
 9. A defrostcycle initiation system as set forth in claim 8 wherein said evaporatoris a coiled refrigerant line having a plurality of end turns, saidapparatus including a compartment enclosing said end turns of theevaporator, said end turn compartment being in communication with saidmain flow path on the upstream side of the evaporator and being incommunication with the enclosure via said first opening whereby withsaid evaporator substantially free of frost build-up at least part ofsaid air flows from said auxiliary air inlet into said main flow path onthe upstream side of the evaporator via said enclosure and said end turncompartment, and with said evaporator at least partially blocked byfrost, the flow of air in said auxiliary flow path is reversed so thatair from the main flow path on the upstream side of the evaporatorenters the end turn compartment thereby to be chilled by the evaporatorend turns and then enters said enclosure thereby to cold-bias saidsensing means and initiate a defrost cycle.
 10. A defrost cycle systemas set forth in claim 7 wherein said air conditioner includes acompressor and wherein said sensing means is a bimetal thermostat, saidthermostat being operable to initiate operation of said compressor uponsensing air at a first predetermined temperature and to terminateoperation of the compressor upon sensing air at a second and lowerpredetermined temperature, whereby with said evaporator substantiallyfree of frost and with said thermostat sensing the room air temperaturesaid thermostat initiating operation of the compressor upon thetemperature of the room air rising to said first predeterminedtemperature and said thermostat terminating operation of said compressorupon the temperature of the room air dropping to Said secondpredetermined temperature.
 11. A defrost cycle initiation system as setforth in claim 1 wherein said refrigeration apparatus includes arefrigeration system comprising a compressor and an evaporator, thelatter constituting said cooling means, said sensing means beingoperable to initiate operation of the compressor upon sensing a firstpredetermined temperature and to terminate operation of the compressorupon sensing a second and lower predetermined temperature, whereby withsaid evaporator substantially clear of frost the temperature sensingmeans senses the temperature of air from the refrigerated zone so thatupon the temperature of the air in the refrigerated zone rising to saidfirst predetermined temperature said sensing means initiates operationof the compressor, and upon the temperature of the air in therefrigerated zone falling to said second predetermined temperature saidsensing means terminates operation of the compressor and thus maintainsthe temperature of the air in the refrigerated zone between saidcompressor start and compressor stop temperatures.
 12. A defrost cycleinitiation system as set forth in claim 11 wherein said refrigerationapparatus is a room air conditioner and said temperature sensing meansis a bimetal thermostat.
 13. A defrost cycle initiation system as setforth in claim 1 wherein said refrigeration apparatus is a room airconditioner and said temperature sensing means is a bimetal thermostat.